We performed electroretinograms (ERG) on both eyes of 11 diabetic patients before and one month after panretinal photocoagulation for diabetic retinopathy. Each patient had one eye treated with argon laser and the fellow eye treated with the xenon arc photocoagulator. After photocoagulation the ERG was symmetrically reduced unless the retinal area burned with xenon arc was greater than twice the retinal area burned in the fellow eye by the argon laser photocoagulator.